An electric current measuring apparatus is known in which the Faraday effect of the optical fiber is used and which has various advantages, such as a small size, flexibility, electromagnetic noise resistance, long-distance signal transmission, and a withstand voltage, and the like. As an example of the electric current measuring apparatus, Patent document 1 discloses a reflective electric current measuring apparatus which uses the Faraday effect, that is, uses the rotation of the polarization plane of light by the action of a magnetic field.
FIG. 21 shows an electric current measuring apparatus 100 shown in FIG. 18 of Patent document 1, WO 2006/022178 (the reference numerals in the drawings of Patent document 1 are changed for description hereinafter). The electric current measuring apparatus 100 is a reflective electric current measuring apparatus 100 which uses a lead-glass fiber as an optical fiber 101 for a sensor, and a mirror 102 is disposed at the other end of the optical fiber 101 for a sensor. The optical fiber 101 for a sensor is provided the around the periphery of an electrical conductor 103, through which an electric current to be measured flows, to detect the measured electric current. Basically, while linearly polarized light incident from one end of the optical fiber 101 for a sensor is reciprocating by the mirror 102, the Faraday rotation angle of linearly polarized light rotating by the magnetic field of the measured electric current is detected.
Reference numeral 104 represents a light source, reference numeral 105 represents a circulator, reference numeral 106 represents a polarization separating unit, such as calcite, reference numeral 107 represents a Faraday rotator having a permanent magnet 107a and a ferromagnetic crystal 107b such as YIG, reference numerals 108a and 108b represent photodiodes (PD), reference numerals 109a and 109b represent amplifiers (A), reference numerals 110a and 110b represent band pass filters (BPF), reference numerals 111a and 111b represent low pass filters (LPF), reference numerals 112a and 112b represent dividers for obtaining the ratio of an alternating current component and a direct current component of an electrical signal, reference numeral 113 represents a polarity inverter, and reference numeral 114 represents a multiplier. Reference numeral 115 represents an optical system, and reference numeral 116 represents a signal processing circuit.
Of linearly polarized light which is emitted from the light source 104 and separated into an ordinary ray and an extraordinary ray by the polarization separating unit 106, linearly polarized light corresponding to the ordinary ray passes through the ferromagnetic crystal 107b and is then incident on the optical fiber 101 for a sensor. Light is reflected by the mirror 102, passes through the optical fiber 101 for a sensor and the ferromagnetic crystal 107b again, and is incident on the polarization separating unit 106.
The polarization plane of linearly polarized light rotates when linearly polarized light passes through the ferromagnetic crystal 107b and the optical fiber 101 for a sensor. Thus, linearly polarized light is separated into polarized light components in two orthogonal directions by the polarization separating unit 106. The separated polarized light components are respectively guided to light receiving elements 108a and 108b serving as photodiodes by the circulator 105 and the polarization separating unit 106 of FIG. 21.
A current or voltage proportional to the intensity of received light is output from each of the photodiodes 108a and 108b as an electrical signal. The electrical signals pass through the amplifiers 109a and 109b, and are then separated into an alternating current component and a direct current component by the BPFs 110a and 110b and the LPFs 111a and 111b. The ratio of the alternating current component and the direct current component is obtained by the dividers 112a and 112b. With regard to an output signal from the divider 112a, the polarity is inverted by the polarity inverter 113. The average of signals Sa and Sb output from the polarity inverter 113 and the divider 112b is obtained by the multiplier 114, and the average is output as the measurement value Sout of the measured electric current of the electric current measuring apparatus 100.
As the Faraday rotator 107 provided on the optical fiber 101 for a sensor, a Faraday rotator having the Faraday rotation angle of 22.5° at the time of magnetic saturation is used (for example, see Patent document 2, WO 2003/075018).